Process for applying a powder coating

ABSTRACT

A process for coating a substrate comprising the sequential steps of (a) providing a transfer sheet provided with a printed powder coating, (b) applying the transfer sheet onto the substrate with the powder coating in contact with the substrate, (c) removing the transfer sheet from the powder coating and (d) curing the powder coating on the substrate.

The present invention pertains to a process for applying a powdercoating onto a substrate.

Conventionally, powder coatings are applied onto substrates using sprayguns or fluidised beds. Both these processes have the disadvantage thatthey can only deposit powder coatings in a single colour on the entiresubstrate. Any lettering or other decoration will have to be applied ina separate process step.

U.S. Pat. No. 5,955,152 provides an alternative method for applying apowder coating onto a substrate. This reference describes a methodwherein a powder coating is applied onto a substrate via anelectrostatic printing process comprising forming an electrostaticlatent image on a light-sensitive body, developing the electrostaticlatent image by making a thermosetting powder coating material adhereunder electrostatic force to the light-sensitive body, transferring thepowder coating material on the light-sensitive body onto the substrateto be coated, and curing the powder coating in such a way that itadheres to the substrate. The reference also describes a method whereinthe powder coating material on the light-sensitive body is transferredonto a transfer sheet, and the powder coating material is cured as it ispressed against the substrate to be coated, thereby baking the powdercoating material onto the substrate; followed by peeling the transfersheet from the cured coating layer.

While the transfer sheet process described in U.S. Pat. No. 5,955,152makes it possible to deposit powder coatings with different colours ontoa single substrate in a single step, it appears that it still has somedisadvantages.

In the first place, it appears that when the transfer sheet is used toapply a decoration on part of a coated substrate, e.g., a logo onto analready coated refrigerator door, the decoration has an added-onappearance which is not attractive.

Further, the application of adjoining transfer sheets, for example whenproviding a pattern on a very large substrate, is difficult with themethod described in this reference. If the transfer sheets are appliedsimultaneously it is difficult to properly adjoin the patterns, becausethe transfer sheets are in the way. When the transfer sheets are appliedsequentially, that is, one sheet is applied, the coating is cured, thetransfer sheet is removed, and the second sheet is applied, good resultswill also not be obtained because the different patterns will besubjected to different curing conditions.

Further, it sometimes occurs that during curing the powder coating layeris fixed not only to the substrate to be coated, but also to thetransfer sheet in such a manner that the transfer sheet cannot beremoved without damaging the decoration applied via the powder coatinglayer. Because the powder coating layer at that point in time is alreadycured, the decoration cannot be removed anymore. This problem can bealleviated by removing the transfer sheet when the coating layer isstill warm, but this places restrictions on process flexibility whichare not always desired.

A further disadvantage of the process described in this reference isthat it places stringent requirements on the properties of the transfersheet, as the transfer sheet should be able to withstand the conditionsprevailing during curing of the powder coating layer without deforming,tearing, melting, or showing any other behaviour which woulddetrimentally affect the properties of the final coating or thepossibilities for removing the transfer sheet. U.S. Pat. No. 5,955,152mentions Teflon™ films and heat-resistant silicon-based films, which arequite expensive.

These problems are solved by the process of the present invention. Thepresent invention provides a process for coating a substrate comprisingthe sequential steps of providing a transfer sheet provided with aprinted powder coating, applying the transfer sheet onto the substratewith the powder coating in contact with the substrate, removing thetransfer sheet from the powder coating, and curing the powder coating onthe substrate.

A characterising feature of the process of the invention is that thetransfer sheet is removed from the powder coating before the powdercoating is cured. This leads to a number of advantages.

In the first place, it was found that when the powder coating is curedonly after the transfer sheet is removed, the surface properties of thecured powder coating will mach those of any underlying coating layer.This means that even when only part of a substrate is provided with adecoration via a transfer sheet, the decoration will form an integralpart of the coated substrate, which makes for a much more sophisticatedappearance. Further, also when the entire surface of a substrate isprovided with a powder coating decoration via the method according tothe invention, the resulting decoration has more depth and character ofimage as compared to the relatively flat results obtained via the methoddescribed in U.S. Pat. No. 5,955,152.

In the method of this invention it is also easy to decorations adjoiningeach other. This can be done by repeating the deposition part of theprocess, comprising the steps of applying the transfer sheet onto thesubstrate with the powder coating in contact with the substrate andremoving the transfer sheet from the powder coating as many times asdesired, and then subjecting the entire decorated substrate to a curingstep. As each transfer sheet is applied after the previous transfersheet has been removed, matching the patterns of the adjoining sheets isnot hard. As all decorations will be cured simultaneously, the effectsassociated with different curing conditions will not occur.

Further, if the powder coating layer is damaged during removal of thetransfer sheet, the fact that the powder coating is at that point intime not cured may make for easier removal of the powder coating, sothat the substrate can be cleaned and the transfer re-applied.

Additionally, the material of which the transfer sheet is made can beoptimised for printing and transfer properties, without having towithstand the conditions prevailing during curing, making a widerselection of materials available for use in the process.

The process according to the invention allows the application of anydesign in one or more colours onto a substrate in a single step.

Further advantages associated with the process of the present inventionwill become apparent from the document below.

In the first step of the process according to the invention, a transfersheet with a printed powder coating is provided. The printing processcan be any conventional printing process via which powder materials areapplied onto a substrate. Suitable printing processes include thosecomprising the following steps. In a first step an electrostatic latentimage is formed on a light-sensitive body. In a second step theelectrostatic latent image is developed by letting the powder coatingadhere under electrostatic force to the light-sensitive body. Then, thepowder coating material on the light-sensitive body is transferred ontothe transfer sheet. For more details on suitable printing processesreference is made to U.S. Pat. No. 5,955,152.

In one embodiment a plurality of powder coatings of different coloursare sequentially printed onto the transfer sheet, for example by meansof the latent-image forming step, developing step and transfer step.Then the plurality of powder coatings are transferred onto thesubstrate, followed by removal of the transfer sheet and curing. Thisprovides an easy method of providing a substrate coated with a designcontaining multiple colours. It should also be noted that the use of atransfer sheet makes for a more risk-free coating process than directcoating onto the substrate, in that a miss-print on a transfer sheet isless expensive than a miss-print on a final substrate.

In one embodiment that substrate is provided with a coating before it issubjected to the process according to the invention. This base coat mayhave any color, but especially when it is white, base colour of thesubstrate will not influence the colour of the final coated substrate.The base coat may be a fully cured coating layer (preferably a powdercoating), it may have been only partially cured, or it may be a melteduncured powder coating. The latter is preferred.

In one embodiment the plurality of types of powder coating materialsinclude powder coating materials of the standard printing colours cyan,yellow, magenta, and black. This will allow the provision of any desiredcolour on a white substrate.

In one embodiment of the present invention the powder coating on thetransfer sheet is submitted to a temporary fixing step in which thepowder coating is heated to a temperature below its curing temperaturein such a way that it is temporarily fixed on the transfer sheet. Thepowder coating is heated sufficiently to induce some flow between thepowder coating particles, but insufficiently to cause any substantialcuring of the powder coating. If so desired, the powder coating ispressed to the transfer sheet during the temporary fixing step, e.g., byusing rollers.

If so desired, a protective sheet may be applied onto the powder coatinglayer to protect the coating layer against being damaged. This may be ofinterest for the powder coating as such and for the powder coating layerafter it has been submitted to a temporary fixing step.

Any suitable material can be used for the protective sheet, as long asit can be removed from the powder coating layer without damaging it.Suitable materials will be evident to the skilled person and includeconventional release paper such as waxed paper of grease-proof paper.

The next step in the process according to the invention is applying thetransfer sheet on the substrate with the powder coating in contact withthe substrate.

The transfer sheet may be applied to a flat surface, but may also befolded around a three-dimentional structure.

How this process step should be carried out will be evident to theskilled person. It may be appropriate to press the transfer sheetagainst the substrate using rollers or other pressure-generatingapparatus. It is also possible, as is known in the art, to cover thesubstrate with the transfer sheet and then apply a vacuum to press thetransfer sheet to the substrate. To increase the adherence of the powdercoating to the substrate is necessary for the powder coating to beheated above its melt temperature while in contact with the substrate.In general, the conditions during this transfer step should be chosensuch that no substantial curing of the powder coating is observed. Ingeneral this means that this process should be done quite rapid and thetemperature should not be chosen too high. In general, the powdercoating is heated to a temperature of at least 2° C. above the melttemperature, preferably at least 5° C., more preferably at least 10° C.In general, a higher temperature is preferred as this will increase theadherence of the powder coating to the substrate.

Since it is not intended to cure the powder coating at this stage, thetemperature of the powder coating should be kept substantially below itscuring temperature, e.g., at least 2° C. below its curing temperature,preferably at least 5° C., more preferably at least 10° C.

Then, the transfer sheet is removed from the powder coating. This shouldbe done at a temperature below the melt temperature of the powdercoating, e.g., at least 2° C. below the melt temperature, preferably atleast 5° C., more preferably at least 10° C.

The powder coating layer will be retained on the substrate.

Finally, the powder coating layer is subjected to a curing step in whichthe powder coating is cured to provide internal integrity and adhesionto the substrate. Preferably, heat curing is used. Curing powdercoatings is standard technology and requires no further elucidation.

In a further embodiment, another powder coating layer is applied using atransfer sheet, before the curing step of the first coating layer thatwas applied via a transfer sheet powder coating. In such processsequential layers of powder coating are applied using a transfer sheetbefore the final curing step. This embodiment enables the preparation ofa powder coating layer on a substrate with various textures and/or thebuilding of a 3-dimensional structure in the coating layer on thesubstrate.

This embodiment can be characterised as a process having the sequentialsteps of

-   1. providing a transfer sheet provided with a printed powder coating-   2. applying the transfer sheet onto the substrate with the powder    coating in contact with the substrate-   3. removing the transfer sheet from the powder coating-   4. applying another transfer sheet provided with a printed powder    coating onto the previously coated substrate with the powder coating    in contact with the previously coated substrate-   5. removing the transfer sheet from the powder coating.-   6. curing the powder coating on the substrate

Further steps 4 and 5 can be introduced once initial steps 4 and 5 havebeen completed.

The powder coating used in the process according to the invention is athermosetting powder coating composition which comprises a resin and acuring agent therefore. This differentiates powder coatings fromconventional toners: the latter do not contain a curing agent.

The powder coating may further comprise one or more of the conventionalpowder coating additives like pigments, fillers, and opacifyers.

The present invention is not limited to the embodiments discussed above.For example, there is no limitation on the colours of the powder coatingmaterials.

If so desired, the substrate may be pre-coated with liquid coatings orpowder coatings via conventional methods. It may be preferred for thesubstrate to have been provided with a coating layer with a thickness of1-200 microns, in particular 10-150 microns. The coating layer being apowder coating layer may be particularly preferred.

1-8. (canceled)
 9. A coated substrate obtained by a process comprising the sequential steps of: providing a transfer sheet provided with a printed powder coating; applying the transfer sheet onto the substrate with the powder coating in contact with the substrate; increasing the adherence of the powder coating to the substrate by heating said powder coating to a temperature above its melt temperature but substantially below its curing temperature; removing the transfer sheet from the powder coating; and curing the powder coating on the substrate after the step of removing the transfer sheet from the powder coating.
 10. The coated substrate of claim 9 wherein the coating layer has a thickness of 10-150 microns. 